WordPress MU

I am curious to know if there is a theme out there that will allow me, the MU admin, to let the bloggers who come to the site - the opportunity to customize the blog header and maybe styles. I see at wordpress.com that they offer this I just am not sure how I can do it but still maintain control over styles so they don't mess up the wrapper on our site. I know people like to sometimes create their own banners and change them up with the seasons.

Does anyone know if this is a plugin or something wordpress has done on the backend?

Thanks!
Helen

It's integrated into some themes. there are themes that are out there with custom headers, but you can add the stuff to the themes you have too.

http://boren.nu/archives/2007/01/07/custom-image-header-api/